This paper reports a detailed thermodynamic analysis of the GaAs/Cl and InP/Cl chemical systems, which are of interest in the technological processing of these III–V materials. The thermodynamically predicted dependence of the steady state chemical etching on both Cl2pressure and temperature is derived assuming Langmuir free evaporation from the surface. The chemical potential data base used in this thermodynamic analysis has been checked for accuracy against all available vapor pressure measurements in the literature. The thermodynamically predicted chemical etching is compared to the etching observed in a Cl2plasma. This approach shows promise in semiquantitative modeling of the dependence of these reactions on both the temperature and Cl2pressure. In addition, changes in the surface morphologies resulting from plasma etching appear to be correlated with the thermodynamically predicted transitions of various compounds on the surface.